The present application claims priority to Japanese Application No. P2000-067209 filed Mar. 7, 2000, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a multilayer optical disk having an information recording layer formed of a phase-change material serving as a recording material, and more particularly to a method of initializing such a multilayer optical disk.
2. Description of the Related Art
With advances in information and communication device technology, a need has arisen for an optical disk having a greater storage capacity.
In the art of two-layer digital versatile disks (DVDs), a storage capacity of 8.5 Gbytes has been achieved. However, the achieved capacity is not high enough to replace a home video recorder with an optical disk recorder or to record a high-vision broadcasting program.
In order to record a 4-hour broadcasting program according to the current NTSC standard using an optical disk instead of a conventional home video recorder, the optical disk has to have a storage capacity of 8 Gbytes or higher. In order to record a 3-hour high-vision broadcasting program, a storage capacity of 20 Gbytes or higher is required.
With future advances in the digital information and communication technology used in our societies, there will arisen a need for a recordable optical disk having an increasingly higher storage capacity.
As described above, although in read-only optical disks, a storage capacity of 8.5 Gbytes has already been achieved by means of a two-layer DVD disk, the maximum storage capacity is as small as 5.2 Gbytes in commercially available recordable optical disks. The commercially available recordable optical disks are of the single-layer type having only one information recording layer.
In recent years, significant efforts have been made to develop an optical disk including two recordable information layers, and some of them can be found in publications.
All proposed optical disks including two recordable layers are of the phase-change type using a phase-change recording material.
The phase-change optical disk is produced by depositing a reflecting metal film, a thin dielectric film, and a thin recording material film using a sputtering apparatus. Because the as-deposited thin recording material film is in an amorphous state, it is necessary to initializing the phase-change optical disk so as to convert it into a crystal state before recording information.
In general, the initialization of the phase-change type optical disk is performed using an apparatus (initializing apparatus) having functions similar to those of a drive device (drive) used to record/reproduce information on/from the optical disk, wherein light having a different wavelength from that used to record/reproduce information is used. The wavelength used in the initialization is generally in the infrared range, because laser diodes having high power capacity such as 1 W or 2 W are available in the infrared range.
The initialization using such an initializing apparatus is described in further detail below. First, a laser beam used for initialization is focused on an information recording layer through a condensing lens called an objective lens. The focused laser beam has a size of about 50 xcexcm to 100 xcexcm in a radial direction of the optical disk and about 1 xcexcm in a direction along a track.
The temperature of the thin recording material film in the recording layer is raised near its melting point and then cooled at a rather slow rate so that the heated and cooled region of the thin recording material film is brought into a crystal state.
When a first recording layer of two information recording layers formed of a phase-change recording material is initialized using the above-described initializing apparatus, the initializing light focused upon the first recording layer passes through the first recording layer and further through a transparent layer (intermediate layer) disposed between the first recording layer and the other recording layer (second recording layer) and is then reflected by the second recording layer. The reflected light again passes through the intermediate layer and strikes the first recording layer. As a result, interfering occurs between the light which is focused upon the first recording layer and strikes it directly and the light striking the first recording layer after passing through the first recording layer and being reflected by the second recording layer.
The thickness of the intermediate layer has a variation on the order of several xcexcm. The variation in the thickness of the intermediate layer causes a variation in the phase of two light rays interfering with each other on the first recording layer across the disk surface. Thus, the intensity of the laser beam striking the first recording layer varies across the disk surface. As a result, the initialization state of the first recording layer varies across the disk surface.
In the phase-change optical disk, the characteristic of a recorded/reproduced signal depends upon the initialization state, and a good characteristic of a recorded/reproduced signal cannot be obtained if there is a variation in the initialization state across the disk surface.
One possible method of reducing the initialization variation in the first recording layer due to the optical interlayer interference is to reduce the reflectance of the second recording layer for light with the wavelength used in the initialization. Another possible method is to reduce the transmittance of the first recording layer for light with the wavelength used in the initialization.
However, the multilayer structure of the information recording layer is generally designed such that optical thin films thereof are optimized so as to obtain a best characteristic of the recorded/reproduced information signal. If the reflectance or the transmittance for the wavelength used in initialization is taken into account in the design of the information recording layer, it becomes difficult to achieve the best characteristics of the information signals recorded/reproduced on/from the respective information recording layers. Although it would be desirable to reduce the transmittance of the first recording layer or reduce the reflectance of the second recording layer for the initialization wavelength without causing degradation in the characteristic of the information signals recorded/reproduced on/from the respective information recording layers, realization of such a reduction is very difficult.
In view of the above, it is an object of the present invention to provide a multilayer optical disk which can be initialized with a less initialization variation due to interference of light without degradation in the characteristic of signals recorded/reproduced on/from respective recording layers. It is another object of the present invention to provide a method of initializing such a multilayer optical disk.
According to an aspect of the present invention, to achieve the above objects, there is provided a multilayer optical disk comprising at least a first information recording layer and a second information recording layer formed in a multilayer structure with an intermediate layer disposed therebetween, the multilayer structure being formed on a substrate, wherein initialization of the first information recording layer is performed by illuminating the first information recording layer with initializing light applied from the side of the first information recording layer, the multilayer optical disk being characterized in that: the fist information recording layer is formed of a phase-change material serving as a recording material; and the numerical aperture NA of an objective lens used for illumination with the illuminating light, the longitudinal length L of the beam size of the initializing light, the thickness D of the intermediate layer, and the ratio R1 of the amount of light striking the first information recording layer after passing through the first information recording layer and being reflected by the second information recording layer to the amount of light incident on the first information recording layer satisfy the following relation for a wavelength xcexi of the initializing light:
Lxc3x97R1xe2x89xa625[1xe2x88x92exp(xe2x88x920.0004xc3x97NA4xc3x97D2)]. 
According to another aspect of the present invention, there is provided a multilayer optical disk comprising at least a first information recording layer and a second information recording layer formed in a multilayer structure with an intermediate layer disposed therebetween, the multilayer structure being formed on a substrate, wherein initialization of the second information recording layer is performed by illuminating the second information recording layer with initializing light applied from the side of the first information recording layer, the multilayer optical disk being characterized in that: the second information recording layer is formed of a phase-change material serving as a recording material; and the numerical aperture NA of an objective lens used for illumination with the illuminating light, the longitudinal length L of the beam size of the initializing light, the thickness D of the intermediate layer, and the ratio R2 of the amount of light striking the second information recording layer after being reflected by the second information recording layer and further reflected by the first information recording layer back to the second information recording layer to the amount of light incident on the second information recording layer satisfy the following relation for a wavelength xcexi of the initializing light:
Lxc3x97R2xe2x89xa625[1xe2x88x92exp(xe2x88x920.0004xc3x97NA4xc3x97D2)]. 
According to still another aspect of the present invention, there is provided a method of initializing a multilayer optical disk comprising at least a first information recording layer and a second information recording layer formed in a multilayer structure with an intermediate layer disposed therebetween, the multilayer structure being formed on a substrate, wherein initialization of the first information recording layer formed of a phase-change material serving as a recording material is performed by illuminating the first information recording layer with initializing light applied from the side of the first information recording layer, the method being characterized in that: the illumination with the initializing light is performed such that the numerical aperture NA of an objective lens used for illumination with the illuminating light, the longitudinal length L of the beam size of the initializing light, the thickness D of the intermediate layer, and the ratio R1 of the amount of light striking the first information recording layer after passing through the first information recording layer and being reflected by the second information recording layer to the amount of light incident on the first information recording layer satisfy the following relation for a wavelength xcexi of the initializing light:
Lxc3x97R1xe2x89xa625[1xe2x88x92exp(xe2x88x920.0004xc3x97NA4xc3x97D2)]. 
According to still another aspect of the present invention, there is provided a method of initializing a multilayer optical disk comprising at least a first information recording layer and a second information recording layer formed in a multilayer structure with an intermediate layer disposed therebetween, the multilayer structure being formed on a substrate, wherein initialization of the second information recording layer formed of a phase-change material serving as a recording material is performed by illuminating the second information recording layer with initializing light applied from the side of the first information recording layer, the method being characterized in that: the illumination with the initializing light is performed such that the numerical aperture NA of an objective lens used for illumination with the illuminating light, the longitudinal length L of the beam size of the initializing light, the thickness D of the intermediate layer, and the ratio R2 of the amount of light striking the second information recording layer after being reflected by the second information recording layer and further reflected by the first information recording layer back to the second information recording layer to the amount of light incident on the second information recording layer satisfy the following relation for a wavelength xcexi of the initializing light:
Lxc3x97R2xe2x89xa625[1xe2x88x92exp(xe2x88x920.0004xc3x97NA4xc3x97D2)]. 
When the information recording layers of the multilayer optical disk is subjected to processing called initialization using laser light (with a wavelength of xcexi), the interference which occurs on the first information recording layer between the incident initializing light and the reflected light is suppressed by minimizing the amount of laser light (with a wavelength of xcexi) which strikes the first information recording layer after passing through the first information recording layer and further through the intermediate layer and then being reflected by the second information recording layer relative to the amount of laser light which is focused upon the first information recording layer and which directly strikes it, thereby reducing the variation in the intensity of light on the first information recording layer due to the interference of light and thus achieving uniform initialization over the entire information recording layer.
Similarly, the interference which occurs on the second information recording layer between the incident initializing light and the reflected light is suppressed by minimizing the amount of laser light (with a wavelength of xcexi) which strikes the second information recording layer after being reflected by the second information recording layer and then passing through the intermediate layer and further being reflected by the first information recording layer and again passing through the intermediate layer relative to the amount of laser light which is focused upon the second information recording layer and which directly strikes it, thereby reducing the variation in the intensity of light on the second information recording layer due to the interference of light and thus achieving uniform initialization over the entire information recording layer.
One method of minimizing the amount of light with the initialization wavelength reflected by the second information recording layer is to form the intermediate layer between the first information recording layer and the second information recording layer so that the intermediate layer has a characteristic varying depending upon the wavelength of light.
More specifically, the intermediate layer is formed so as to have transmittance equal to or higher than 95% at the wavelength used for recording/reproducing information but have sufficiently low transmittance at the wavelength used for the initialization so that initialization is performed uniformly. That is, using the above technique, the variation in the intensity of light striking an information recording layer being initialized is suppressed within an allowable range depending upon the required characteristic of the recorded/reproduced information signal.
For example, the wavelength-dependent absorbance of the intermediate layer can be realized by mixing a dye material into the transparent intermediate material. Herein, the dye material preferably has absorbance at the initialization wavelength but has as low absorbance as possible at the wavelength used for the recording/reproducing of information.